The growth of films of nanostructures, such as carbon nanotubes, on substrates can be performed by a variety of methods. The methods include thermal Chemical Vapor Deposition (CVD) where a chemical precursor is converted into a desired material such as a thin film or a deposit of nanostructures. The deposition occurs on a substrate, and the substrate is typically energized (heated) to promote the deposition process. The precursor typically breaks down and a portion of the dissociated precursor binds to the substrate while the remainder leaves the site as waste product. A properly prepared substrate can be used to define the adsorption areas and thus control the locations of growth.
For example, to produce carbon nanotubes by CVD, an appropriate catalyst and precursor are chosen, and after the precursor surrounds the adsorption sites, a carbon nanotube “grows” from the catalyst particles on the substrate. There are numerous methods of producing CNTs by CVD; for example thermal CVD at atmospheric pressure, using a hydrocarbon gas (e.g. C2H2, C2H4, or CH4) and a predeposited arrangement of catalyst nanoparticles. At a relatively high areal density of catalytic growth sites and a relatively high CNT growth rate, a vertically aligned (VA) growth mode is typical whereby the CNTs self-orient perpendicular to the substrate surface due to initial crowding and continue to grow upward in this direction.
Previously, non-contact methods have been used to guide the direction of CNT growth. For example, isolated single wall nanotubes (SWNTs) can be grown to millimeter or centimeter lengths when suspended by gas flow during growth, and application of an electric field during growth can exert a force on a nanotube and/or a catalyst particle, to achieve aligned and direction-controllable growth in plasma-enhanced CVD processes, and to achieve growth of aligned CNTs spanning gaps between microfabricated electrodes. Further, fluid flows and surface tension forces at fluid interfaces can direct post-growth assembly of CNTs into patterns and structures on surfaces.